1. Field of the Invention
This invention relates to electrophotography and more particularly to an improved scanning light exposure control system for use in an electrophotographic reproduction system.
2. Description of the Prior Art
Electrophotographic image reproduction systems have been in existence for a number of years. In general, such systems operate as follows. An imaging element comprising a photoconductive layer that upon exposure to actinic radiation becomes conductive allowing an accumulated charge on the element surface to selectively bleed through a conductive path is first charged with a uniform charge layer by passing such element under a source of ionizing radiation, e.g., a scorotron or other such corona charging device. The charged surface is then exposed to imagewise modulated actinic radiation, rendering the photoconductor layer conductive and discharging the accumulated charge. The term "actinic radiation" is construed to encompass not only photochemical activity but also the photoelectric effects described herein and the like.
In a continuous tone system, as contemplated herein, the amount of charge left on the imaging element surface is inversely proportional to the amount of actinic radiation received by the element. In this manner a pattern of electrostatic charges is produced on the imaging element forming a latent image corresponding to the imagewise modulated actinic radiation incident on the element. The magnitude of the electrostatic charge at any one point on the imaging element is inversely proportional to the intensity of the exposing actinic radiation.
The latent image may now be rendered visible by development using colored particles which preferably bear a static charge and which are attracted to the charge pattern on the imaging element. Depending on the desired result, the colored particles may bear a charge of the same polarity as the charge originally placed on the imaging element or an opposite polarity. If the charge polarities are the same and an appropriate bias electrode is used the colored particles are preferentially attracted to the areas from which the original charge has been bled away, producing a "dark" or "colored" area of intensity proportional to the original exposure. If the charge polarities are opposite, then the areas that received the least exposure to actinic radiation will attract the most particles. In the first instance there is an image reversal; the light tones appear dark and the dark tones appear light. In the second instance the image tones are reproduced the same as the original.
The colored particles may be in dry form or may be supplied in a dispersion in a carrier liquid. Generally referred to as toners, the colored particles or dispersions are well known in the art. Liquid toners tend to produce higher image resolution and are sometimes preferred for that advantage.
Following toning, the image may be viewed as such, dried, fused or transferred onto a receiving element or any combination of the above, as is well known in the art.
A problem in the reproduction of images using electrophotographic systems of the type described above is banding. Banding is the appearance of horizontal, vertical, or both, stripes of lighter or darker density which usually traverse the full width or length of the image. It is a problem particularly with scanning exposure sources which involve repeatedly traversing an information carrying, intensity modulated, beam of actinic radiation over the surface of an imaging element while the imaging element is driven in a direction generally perpendicular to the traversing beam direction.
The cause for these bands is twofold. Horizontal bands, that is, bands extending transversely to the direction in which the imaging element moves, are usually due to drive irregularities. The drive irregularities cause small variations in the speed of the imaging element which cause partial overlap of the beam and result in areas of the imaging element receiving higher or lower exposure to the actinic radiation. Vertical bands, that is, bands extending along the direction of movement of the imaging element, are generally attributed to irregularities in the actinic radiation source. In the case where a cathode ray tube (CRT) is used as an exposure source these irregularities are typically caused by non-uniformities in the phosphor layer used in the CRT to produce a luminous output. Both problems are most visible and distracting when a continuous tone image is being reproduced.
There is, thus, need for a scanning exposure source that produces imaging having reduced banding defects.